Optical head, lazer diode module and optical recording-reproducing apparatus

Abstract

The spot diameter D3 of a side beam 21 imaged on a disc 5 is set to at least 2.5 times as large as that of a main beam. Via this configuration, an output signal from a detector detecting a reflected light via a side beam, that is, a signal representing a different in the output from photo-detection devices split by split lines in the radial contains a negligible track cross component caused by a beam spot crossing a track. The signal substantially contains a DC offset component alone caused by the shift of an objective lens in the radial direction under tracking control. Thus, by subtracting the output signal of the detector from the push-pull signal of the main beam, a tracking signal substantially containing no DC offset.

What is claimed is:  
     
       1. An optical head comprising: 
       a light source for radiating a single laser beam;  
       a diffraction element which splits said single laser beam into a plurality of sub-beams and radiates the plurality of sub-beams on a disc for tracking control;  
       wherein a diameter of said sub-beam on a disc is set to at least 2.5 times as large as that of the main beam.  
     
     
       2. The optical head as claimed in  claim 1 , further comprising a beam splitter, which causes a light coming from said light source to radiate onto the disk and causes a light reflecting from the disk to radiate to a detector. 
     
     
       3. The optical head as claimed in  claim 1 , wherein said diffraction element enlarges the diameter of said sub-beam on said disc. 
     
     
       4. The optical head as claimed in  claim 3 , wherein said diffraction element includes a hologram device designed to give aberration to beams other than a zero-order light. 
     
     
       5. An optical head according to  claim 1 , further comprising a detector for receiving a sub-beam reflected on a disc is split into at least four areas and producing a focusing error signal through arithmetic operation. 
     
     
       6. A laser diode module comprising: 
       a optical head including:  
       a light source for radiating a single laser beam; and  
       a diffraction element which splits said single laser beam into a plurality of sub-beams and radiates the plurality of sub-beams on a disc for tracking control;  
       wherein said diffraction element enlarges a diameter of said sub-beam on a disc to at least 2.5 times as large as that of the main beam.  
     
     
       7. A laser diode module as claimed in  claim 6 , wherein said diffraction element includes a hologram device designed to give aberration to beams other than a zero-order light. 
     
     
       8. A laser diode module according to  claim 6 , further comprising a detector for receiving a sub-beam reflected on a disc is split into at least two areas and producing a focusing error signal through arithmetic operation. 
     
     
       9. An Optical recording/reproducing apparatus comprising: 
       an optical head including a light source for radiating a single laser beam; and a diffraction element which splits said single laser beam into a plurality of sub-beams and radiates the plurality of sub-beams on a disc for tracking control, in that a diameter of said sub-beam on a disc is set to at least 2.5 times as large as that of the main beam,  
       detectors which receive disc-reflected lights of a main beam and sub-beams respectively, each of said detector being split by split line in the tracking direction,  
       wherein arithmetic operation processing on output signals from each detector split by said split line to obtain a tracking signal.  
     
     
       10. The optical recording/reproducing apparatus according to  claim 9 , wherein a DC offset signal component of a tracking signal is removed by subtracting a signal obtained from a side beam via the push-pull method from a tracking signal obtained from a main beam via the push-pull method. 
     
     
       11. An optical recording/reproducing apparatus according to  claim 9 , further comprising a detector for receiving a sub-beam reflected on a disc is split into at least four areas and producing a focusing error signal through arithmetic operation. 
     
     
       12. An optical recording/reproducing apparatus according to  claim 6 , further comprising a detector for receiving a sub-beam reflected on a disc is split into at least two areas and producing a focusing error signal through arithmetic operation.